Electrical power line generation, distribution, and utilization commonly involves polyphase systems, in which several sinusoidal voltage sources equal in magnitude but different in phase are employed. Polyphase systems include two phase and three phase systems. Three phase systems offer economic and operating advantages, and are the most common. The respective voltages of a three phase system are normally generated by the same machine or generator. A power load may utilize all of the phases or a single phase of a polyphase power distribution system. For example, machinery found a in heavy industrial environment frequently draws power from all three phases. Other loads may only be connected across any one of the phases (line and the neutral). The majority of electrical products sold for general use are intended for connection to a single phase.
FIG. 1 shows three voltage sources Va, Vb, and Vc in a three phase system. Each source may be, for example, one hundred volts, as measured across the generating source. Source Va, for example, is one hundred volts, as measured from a voltage point "a" to a point "o." If the generating source Va is taken as the reference with a phase angle of zero (0) degrees, then the phase angle of Vc will be one hundred twenty (120) degrees and the phase angle of Vb will be two hundred forty (240) degrees.
A phasor diagram of the three sources Va, Vb, and Vc is shown in FIG. 2. Shown in the figure are source Va at 0 degrees, source Vc at 120 degrees, and source Vb at 240 degrees. Note that source Vb may also be measured as having a phase angle of negative 120 degrees. Thus, the relationship between each of the sources Va, Vb, and Vc in the three phase system is that each source is 120 degrees out of phase with the other generating sources.
The three sources Va, Vb, and Vc of FIG. 1 may be connected with their respective "o" points tied together to form a Y-connection, as shown in FIG. 3. Alternatively, the three sources Va, Vb, and Vc may be connected with each "o" point joined to a corresponding voltage point of another source, as shown in FIG. 4. The configuration of FIG. 4 is known as a delta-connection.
In the Y configuration connection of FIG. 3, lines which extend from the voltage sources Va, Vb, and Vc form the three phases distribution system. The common point (or neutral) may or may not be distributed as a fourth line. If the neutral is distributed then the system is a four-wire three phase system. In the delta-configuration of FIG. 4, there is no neutral. The four wire three phase system of FIG. 3 is almost universally used to distribute power within commercial buildings, and hence the presently preferred embodiment is described using this configuration. The present invention, however, may also be used with a two phase or delta-distribution system.
In the past, it generally has not been important in a multi-phase system to identify the respective phases of particular power outlets, other than at the time of wiring the building so that the loads could be equally distributed on all the phases. Recently, however, power distribution lines have been used increasingly for data communication, due to the cost savings achieved by avoiding the installation of dedicated communication cabling. By utilizing the power distribution wire, the need for other forms of communication media (e.g., twisted pair, coax cable) may be eliminated, while allowing the transfer of data between devices that are connected to the power line at various outlets throughout a building.
When a polyphase power distribution is used for communication, it is beneficial, and in some cases essential, to identify the phase of various power outlets. This is often necessary since the difficulty of cross phase communication frequently dictates the installation of additional devices to provide more reliable communication between phases.
Power lines may not be ideal for data communication media, since power line characteristics do not readily transmit frequencies needed for useful data rates. The inductance of the power lines and the loading of equipment connected across these power lines causes attenuation of the data signals, for example. Also, appliances such as motors, light dimmers, switching mode power supplies, television (TV) sets, and fluorescent light ballasts couple noise on to the power lines which can degrade data communication signals. Moreover, in buildings that use the three phase four wired system, data signals on one phase are coupled to other phases by means of parasitic inductance and capacitance between distribution lines and within distribution transformers. The present invention relies on the parasitic coupling between phases for transmission of data packets across polyphases distribution system. However, the present invention does not require the degree of parasitic coupling needed by control or data systems, as only small amounts of data need to be successfully communicated with relatively low reliability.
In retrofitting a building for a control network, where the power lines will be used for both AC power and data communications, the above shortcomings can be effectively combated once Phase identification is accomplished. Phase identification involves the determination of the relative phases of power outlets with respect to each other. If upon installation of a power line communication system it is found that there is high signal attenuation between the phases (i.e., insufficient transmitted signal propagated from a reference phase outlet to another outlet of a phase being tested) then appropriate network components or devices may be inserted to overcome the degradation of the signal.